1. Field of the Invention
The present invention relates to an oil deterioration detector which detects deterioration of oil by detecting acidity and/or basicity of oil.
2. Description of the Related Arts
One of conventional methods for detecting deterioration of oil is to measure a potential difference between a reference electrode and a sensitive electrode which are made of different metals, as disclosed in Unexamined Japanese Patent Application No. HEI 3-175350/1991 and Japanese Patent No. HEI 4-62336/1992.
For example, the former discloses a reference electrode made of base metal, such as lead, and a sensitive electrode made of electrically conductive solid material, such as stainless steel having an oxide membrane thereon. Measuring a potential difference between these reference electrode and sensitive electrodes, acidity and/or basicity (hereinafter pH) of oil is detected.
It is estimated, in this related art, that the electric potential of the reference electrode is stable even if the pH value of oil varies, because the base metal such as lead, when used as the reference electrode, mainly causes electrolytic dissolution into the oil. On the contrary, the electric potential of the sensitive electrode, which is made of electrically conductive solid electrode, tends to vary depending on the equilibrium between OH ions absorbed on the electrode surface and H ions representing the pH value of the oil. Furthermore, it is also estimated that the electric potential is stabilized when an oxide membrane is formed on the electrically conductive solid electrode.
In the case where above-described electrodes are incorporated into an oil deterioration detector for an automotive vehicle, these paired electrodes need to be put or dipped into an oil circulating circuit. However, the oil circulating circuit is normally constituted by numerous oil devices made of metal (e.g. carbon steel), such as a pump, an oil filter, an oil cooler, an oil tank, and piping. It means that, from the view point of electrochemistry, these metallic oil devices can perform as one of electrodes. In other words, a total of three electrodes is put or dipped into the oil.
Accordingly, a potential difference is caused between the metallic oil devices and the metallic electrode (reference electrode); thus, a significant amount of electric current flows between the metallic oil devices and the reference electrode. In the same manner, a potential difference is caused between the metallic oil devices and the electrically conductive solid electrode (sensitive electrode); thus, a significant amount of electric current flows between the metallic oil devices and the electrically conductive solid electrode. This phenomenon adversely affects the signal voltage to be detected between the reference electrode and the electrically conductive solid electrode. Especially, to detect a potential difference between these electrodes, this technology requires a potentiometer with an internal resistance (input impedance) of approximately 10.sup.11 which is an extremely high resistance. Hence, if the metallic oil devices--third electrode (earth electrode)--are disposed close to the metallic electrode and/or the electrically conductive solid electrode, it will incur the serious decline of S/N ratio in the detection of the signal voltage.
Using a bath voltage Vs and a bath resistance Rs between the reference electrode and the sensitive electrode, and an input impedance Rin of the potentiometer serving as a measuring circuit, a measured electric potential Vm of the potentiometer is expressed by the following equation. EQU Vm=Vs.multidot.Rin/(Rs+Rin) (1)
As apparent from this equation (1), to increase measurement accuracy--namely, to approximate the measured electric potential Vm to the bath voltage Vs, it is necessary to enlarge the input impedance Rin with respect to the bath resistance Rs so as to allow electric current to flow in the measuring circuit as less as possible. As the oil intervening these electrodes has a volume resistance of 10.sup.8 .OMEGA.-cm, the bath resistance Rs becomes a fairly large value. Accordingly, the input impedance Rin needs to be enlarged in accordance with this bath resistance Rs. In fact, the Unexamined Japanese Patent Application No. HEI 3-175350/1991 discloses a potentiometer having an input impedance of approximately 10.sup.11.
However, realization of such a large resistance range is not easily attained by conventionally existing devices. Furthermore, a measuring circuit having such a large input impedance Rin will increase costs, because it requires a specially designed, sealed, structurally-integrated device to exclude moisture in the air and prevent leaks through the connectors.
Moreover, as it is believed that the pH value of the oil is representative of H ion density in water content involved in the oil, it was difficult to promote the electrochemical reactions around the sensitive electrode and, therefore, an output potential of the sensitive electrode could not be increased sufficiently. Hence, the S/N ratio obtained was insufficient.